Pets still require 25% of the experience that a player would at the same level. Players now require less experience to level between levels 20 and 60, and so our pets also require less experience during those levels.

Pets still do not get any share of the experience when their owner completes a quest. (I didn't expect they would, but it was worth checking.)

As on live, pets do not get experience when they kill something that is grey to the hunter.

The experience that pets get per kill does not appear to have changed.

Generally there are no big changes to pet experience in patch 2.3, except that players will be gaining more experience from quests between levels 20 and 60, and of course pets won't be getting any of that. It's too soon to say if this means hunters will start outleveling their pets during those levels or not, but it could be an issue in the future.

Mania has also discovered some interesting details about the relationship between the pet's level and your own level when it is gaining experience. If you're a hunter leveling a lower-level pet, head on over to her post and check it out.